Marshals of France

Does Petain count since he later became a Marshal? If he does, then I'll vote for Davout.

Anyhow, Davout is one of the best strategists in history and he NEVER lost a battle

And not only that... His dynasty rules Sweden until now. Rare achievement, I would say   
His tatoo was cool for a king (http://www.faktoider.nu/bernadotte_eng.html) 
But his military abilities were doubtfull for many...

It is interesting how Napoleon himself "valued" (in both meanings of this word ) his marshals:


"Napoleon’s favouritism was evident in the very unequal distribution of the dotations over the years. The top four marshals – Berthier with 1 million francs, Masséna 933,000 francs, Davout 817,000 francs and Ney 729,000 francs – received over half the total of 6 million francs. The next four – Soult, Bessières, Lannes and Bernadotte – got between 200,000 and 300,000 francs each. All the rest received less than 200,000 francs, with Saint-Cyr, whom Napoleon respected as a soldier but couldn’t warm to as a man, getting only 30,211 francs".
Andrew Roberts, Napoleon: A Life

My vote is for Masséna... He voted against the Life Consulate and criticized the Moreau trial. That means he received his 933.000 francs for military talents only 

I like St. Cyr though. He wasn't overrated like Jean Lannes.

Lannes was the Napoleon's best friend and one of the bravest marshals

Davout, the Marshal who was so good that he actually had a disastrous impact on Napoleon's Russia Campaign:
In the early stages of the campaign, the main Russian armies were those of Bagration (a hot-headed Georgian prince) and Barclay (aka Q. Fabius "Cunctator" Maximus 2.0), who was a more patient and rational general that preferred not fighting a much stronger and larger opponent (600.000 French+allies vs. 200.000 Russians, if I remember correctly). These two generals led two different armies: Bagration's army was to the south of Barclay's army. Now, we do need to remember that Barclay (whose name was suspicious for the xenophobic, nationalist generals in the Russian army) was criticized for simply giving up land instead of fighting the French in a more heroic battle. Among these generals criticizing Barclay was Bagration.

At some point in the early stages of the campaign, Bagration and Barclay tried to fuse their armies in order to create a more effective and a not seperated fighting force. Davout however, maneuvered his corps in between the armies of Bagration & Barclay, which was not an unwise thing to do in general.

However, if the armies had fused together, then the populara Bagration would've easily swept away the (seemingly unmanly and 'weak') Barclay and would've decided to teach Napoleon and his baguettes a lesson or two. Given the superb state of the French army (it truly was at the top of its fighting power, despite the fact that the army was actually too large), the Russian army could've been easily obliterated by Napoleon, which could render Emperor Alexander to be a bit more lenient to Napoleons demands.

tl;dr: Davout: a Marshal so competent that he 'en passant' prevented a decisive battle (and French victory) in the Russia campaign.

Berthier was what Napoleon needed to win Waterloo